Process and plant for urea production

ABSTRACT

A process for producing urea is disclosed, wherein liquid ammonia and carbon dioxide are reacted in a high-pressure synthesis section ( 100 ), and at least part of the carbon dioxide is fed to said synthesis section ( 100 ) in liquid phase. A plant operating according to said process and a method for modernizing existing plants accordingly are also disclosed.

FIELD OF APPLICATION

The present invention relates to a process and plant for ureaproduction. The invention also relates to a method for modernization ofan existing plant for urea production.

PRIOR ART

According to known art, urea is produced by reacting liquid ammonia andgaseous carbon dioxide (CO2) in a high-pressure reactor, typically avertical, stainless-steel vessel containing a series of trays to improvethe mixing of the reactants. Ammonium carbamate is formed in the liquidphase of the high-pressure reactor as intermediate product, and urea isproduced, also in liquid phase, by dehydratation of said ammoniumcarbamate. The product exiting the reactor is substantially an aqueoussolution comprising urea, carbamate and free ammonia.

In more general terms, a urea plant comprises a synthesis section and arecovery section. The recovery section receives a liquid mixture ofurea, carbamate, ammonia and water from the synthesis section, andprovides an aqueous solution of recycled carbamate and ammonia to thesynthesis section. A urea finishing or purifying section can also beprovided downstream the recovery section.

Most of the current plants for producing urea use the so-calledstripping process, which is aimed to recover most of the carbamatecontained in said aqueous solution leaving the high-pressure (HP)reactor, in a so called high-pressure loop (i.e. at a pressuresubstantially equal to working pressure of the reactor), increasing inthis way the energy efficiency.

Stripping processes are known since decades and substantially includethe carbon dioxide (CO2) stripping process and ammonia stripping (orself-stripping) process. To carry out a stripping process, the HPsynthesis section comprises at least a reactor, a stripper and acondenser.

In a CO2-stripping process, the ammonium carbamate is stripped from theurea solution with the aid of the fresh carbon dioxide feed. Strippingtakes place typically in a vertical, steam-heated tubular heatexchanger, wherein unconverted carbamate dissociates into gaseousammonia and carbon dioxide, which are then recombined in a condenserobtaining liquid carbamate. Said liquid carbamate is recycled to thereactor. The condensation heat is also used to produce low-pressuresteam which is used in downstream purification section, thus minimizingthe energy consumption.

In a self-stripping process, the stripping effect is given by gaseousammonia generated from thermal dissociation of the carbamate solutionproduced in the reactor; hence no stripping agent is required and thegaseous carbon dioxide feed is normally introduced directly into thereactor.

A simplified scheme of the high-pressure loop of a knownammonia-stripping (or self-stripping) process is shown in FIG. 7.

The HP loop essentially comprises a reactor 10, a stripper 11, andshell-and-tube condenser 12. The urea synthesis reaction takes placeinside the reactor 10, producing a liquid mixture of urea, carbamate andammonia, which is fed to the stripper 11 via duct 20. Said stripper 11is, for example, a vertical, steam-heated tube heat exchanger with saidliquid mixture flowing inside tubes.

The liquid phase obtained in stripper 11 is fed to a recovery/purifyingsection (not shown) via duct 21; while vapors exiting the stripper aresent to the condenser 12 via a duct 22. Recycled carbamate coming fromthe recovery section is also fed to the condenser 12, through duct 23,and the liquid formed in said condenser 12, containing recyclecarbamate, is sent to reactor 10.

More in detail, the liquid produced in condenser 12, which contains acertain amount of inert gases, is sent to a separator 13 for removingsaid inerts, and the liquid phase is fed to an ejector 14, via ducts 24and 25 respectively. Said ejector 14 is powered by the fresh ammoniafeed of line 15, so that fresh ammonia and recycled carbamate are sentto the reactor 10 via duct 26.

The carbon dioxide is fed to reactor 10 by flow line 16, with the helpof a multi-stage compressor 17 adapted to raise the carbon dioxidepressure up to the operating pressure of the synthesis loop (over 100bars).

Summarizing, a liquid ammonia input is provided to reactor 10 is throughsaid ejector 14 and line 26, while a gaseous CO2 input is provided tothe same reactor 10 through line 16 and the compressor 17.

In a CO2-stripping plant, gaseous carbon dioxide is fed to the stripperinstead of reactor, via a multi-stage compressor. CO2 acts as astripping agent, which causes decomposition of the carbamate and partialseparation of the free ammonia. Gaseous phase exiting the stripper issent to a condenser, which produces an aqueous solution and a vapourstream, including the recycled carbamate as well as the ammonia andcarbon dioxide feed, which are sent to reactor.

The above described plant configurations are open to many modifications,but according to known art the carbon dioxide is always fed in gaseousstate to the stripper or the reactor of the synthesis section. In otherwords, the prior-art teaches to provide a gaseous carbon dioxide feed tothe synthesis section, i.e. to the stripper of a CO2-stripping plant orinto the reactor itself of a self-stripping plant.

SUMMARY OF THE INVENTION

The problem underlying the invention is to improve the energy efficiencyof known process and plant for urea production. Energy efficiency, infact, is penalized by the various components and/or auxiliariesrequiring energy, and inter alia by the multi-stage compressor of thegaseous CO2.

The basic idea underlying the invention is that at least part of thecarbon dioxide is fed to the synthesis section in liquid phase.

Hence, the above stated problem is solved with a process for producingurea wherein liquid ammonia and carbon dioxide are fed to a synthesissection and reacted into said section to form urea, characterized inthat at least part of the carbon dioxide is fed to said synthesissection in liquid phase.

According to a first embodiment of the invention, a part of carbondioxide feeding to the synthesis section is in liquid phase and theremaining part of said carbon dioxide feeding is gaseous.

According to a second embodiment of the invention, the full carbondioxide input to the synthesis section is in liquid phase, and no inputof gaseous CO2 is provided to the synthesis section. This secondembodiment is applicable to a self-stripping process, wherein no gaseousCO2 is required for stripping.

The synthesis section of a urea plant generally comprises at least areactor, a stripper and a condenser forming a high-pressure loop.According to embodiments of the invention, a liquid CO2 is fed to thereactor and/or to the condenser of said high-pressure loop of thesynthesis section. Feeding liquid CO2 to the reactor and/or to thecondenser is preferred because said components, during operation,already contain liquid.

More in detail, according to an embodiment of the invention, the fullliquid amount of CO2 is directed to the reactor. According to anotherembodiment, the full liquid amount of CO2 is directed to the condenser;according to still another embodiment, the liquid amount of CO2 ispartly directed to the reactor and partly directed to the condenser. Inall the above embodiments, there can be a further input of gaseous CO2.In a self-stripping process, said further input of gaseous CO2 isoptional and preferably directed to the reactor; in a CO2-strippingprocess said gaseous CO2 is required and directed to the stripper.

According to another aspect of the invention, liquid carbon dioxideinput is mixed with at least part of the liquid ammonia input; theresulting mixture is then fed to the reactor and/or condenser of thesynthesis section.

An object of the invention is also a plant for producing urea with theabove process, said plant comprising at least:

-   -   a synthesis section;    -   feeding means providing an input of fresh ammonia and an input        fresh of carbon dioxide to said synthesis section;        characterized by said feeding means being adapted to feed at        least part of said input of carbon dioxide to said synthesis        section in liquid phase.

Preferably, the feeding means comprises mixing means disposed to mix atleast part of the liquid ammonia with the liquid carbon dioxide, andfeed the so obtained liquid mixture to the appropriate component of thesynthesis section.

In a preferred embodiment, said mixing means comprise a so-calledT-mixer or a nozzle; preferably said nozzle has a portion with separatecoaxial ducts for liquid carbon dioxide and liquid ammonia, and a secondportion acting as a mixing zone for said liquid carbon dioxide andammonia. More in detail, the mixer has an internal duct with aconvergent outlet substantially corresponding to a convergent portion ofthe external duct, thus obtaining a mixing zone with decreasing crosssection in the axial direction. Said mixing zone of the nozzle isfollowed by a constant cross-section portion and a divergent portion, toslow down the liquid.

Another object of the invention is a method for improving efficiency ofan existing plant for producing urea, said method being characterized byproviding further means adapted to feed at least part of the carbondioxide input to the synthesis section in liquid phase.

According to one embodiment of the above method, a CO2-stripping unit isrevamped maintaining the existing gaseous CO2 feeding means to thestripper, and providing further liquid carbon dioxide feeding meansdirected to the reactor and/or to the condenser of the HP loop.

In another embodiment, a self-stripping unit is revamped replacing theexisting gaseous CO2 feeding means with the inventive liquid CO2 feedingmeans. These lasts can be directed o the reactor and/or to the condenserof the HP loop.

Preferably, the liquid carbon dioxide feeding means used in said methodfor improving efficiency of an existing urea plant comprise a mixer asdefined above. According to equivalent aspects of the invention, the newfeeding means are provided to feed the liquid carbon dioxide to thereactor and/or to the condenser of the synthesis section.

In all above embodiments, suitable means to liquefy and pump the carbondioxide can be provided, according to per se known art.

It should also be noted that reaction, stripping and condensation can beequally carried out in a single unit or more unit in parallel, accordingto the needs.

The invention has many advantages over the prior art.

It has been found that feeding the synthesis section with at least partof the carbon dioxide in liquid state yields a surprising improvement inthe energy efficiency of the process.

It should be noted that the urea forming reaction takes place in theliquid phase containing carbamate. In prior art processes and plants,liquid ammonia and gaseous CO2 entering the reactor, namely the reactionzone inside the reactor, are mixed with the objective to create anintimate contact between the two phases (that is liquid and gas) andfavor the mass and heat exchange between said reactants. However, yieldof reaction is limited by the rate of mass transfer from the gas phaseto the liquid phase, wherein reaction actually takes place.

The invention overcomes this limitation by providing both reactantsammonia and CO2 in a liquid phase, the yield being no longer limited bythe gas-to-liquid mass transfer. In other words, the reactants are mixedin a more effective manner inside the reaction zone, with advantagesalso in terms of the reaction yield.

It should also be noted that energy consumption of the multistagecompressor of the carbon dioxide (which is indispensable in the priorart) is reduced or even avoided, as pumping a liquid CO2 requires lessenergy than compression of a gaseous CO2. In a CO2-stripping plant theamount of gaseous CO2 to be compressed is reduced, as part of the CO2 isfed in a liquid state; in a self-stripping unit the compressor is nolonger necessary and the full CO2 input can be pumped in liquid form tothe synthesis section. Reduced energy consumption compensates foradditional energy and equipment required for liquefying and pumping theCO2.

Further features and advantages of the present invention will appearmore clearly from the following non-limiting description of embodimentsthereof, made with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general block scheme of a urea plant according to theinvention.

FIGS. 2 and 3 are simplified schemes of the high-pressure loop of aself-stripping plant for producing urea, according to embodiments of theinvention.

FIGS. 4 and 5 are simplified schemes of the high-pressure loop of aCO2-stripping plant for producing urea, according to embodiments of theinvention.

FIG. 6 is a simplified cross section of a nozzle for feeding a mixedflow of liquid ammonia and carbon dioxide, according to a preferredaspect of the invention.

FIG. 7 is a simplified scheme of the high-pressure loop of a knownammonia-stripping (or self-stripping) process.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, a urea plant comprises a synthesis section 100operating at high pressure, a recovery section 200 and optionally afinishing section 300. FIG. 1 refers in the most general way to a ureaplant, such as self-stripping, CO2 stripping or other.

Synthesis section 100 is fed through input line 101 with fresh liquidammonia and is also fed through line 102 with fresh liquid carbondioxide. Another input line 103 of gaseous CO2 can be provided ifnecessary, e.g. if the plant operates according to a CO2-strippingprocess and section 100 comprises a stripper which receives an input ofgaseous carbon dioxide.

Line 102 is connected to suitable means for obtaining liquid carbondioxide, which are per se known and thus not described in detail.

Ammonia and carbon dioxide are the reactants of the urea producingreaction, which is carried out in said synthesis section 100. An aqueoussolution comprising urea, carbamate and unreacted ammonia is obtained insynthesis section 100 and fed to recovery section 200 via line 104; saidsection 200 obtains a solution comprising recycled carbamate andammonia, which is recycled via line 105 to the synthesis section.

Said recovery section 200 is connected via flow lines 201, 202 to thefinishing section 300, wherein urea is produced and a solution ofcarbamate and ammonia is sent back to the recovery section. Purifiedurea U is discharged through line 301.

Sections 200 and 300 are conventional and are not essential for theinvention, and will not be described in detail.

Referring to FIG. 2, the main components of synthesis section 100 of aself-stripping plant, according to one embodiment of the invention, areshown.

The HP loop comprises essentially a reactor 110, a stripper 112 and acondenser 114. Stripper 112 is for example a vertical, steam-heatedshell-and-tube heat exchanger, while condenser 114 is a horizontalshell-and-tube heat exchanger.

A solution substantially comprising urea, carbamate and unreactedammonia is produced in the reactor 110 and is sent from top of saidreactor 110 to the stripper 112, via a flow line (or duct) 120.

Liquid phase exiting the stripper 112, comprising carbamate and urea, issent via flow line 121 to a recovery/finishing section, not shown.Gaseous phase from stripper 112 is sent to the condenser 114, throughline 122. Recycle carbamate coming from the recovery section is also fedto the condenser 114, via line 123.

The output of condenser 114 is a liquid mixture, apart from a fewinerts, which is sent to a carbamate separator 116 (flow line 127) andrecycled to reactor 110 by means of an ejector 118 powered by the liquidammonia feed. Inerts are vented from separator 116 through line 129, andthe liquid phase is sent to ejector 118 via flow line 128, said ejectoralso receiving the fresh ammonia from line 101. A flow substantiallycomprising fresh ammonia and recycled carbamate is then fed to reactor110 via line 126.

A liquid carbon dioxide is fed to reactor 110, from input line 102 andvia line 124. A further flow line 125 provides liquid CO2 input to thecondenser 114, which is mixed in the tube-side of the condenser with thegaseous phase coming from the stripper and the recycle carbamate. Hence,liquid CO2 is fed partly to the reactor 110 via line 124, and partly tothe condenser 114 via line 125 in parallel to line 124.

In other embodiments (not shown) the full amount of liquid CO2 is fedeither to the reactor 110 or to the condenser 114.

A further input of CO2, but in a gaseous phase, can also be provided toreactor 110. In self-stripping units, however, it is preferred to feedall the carbon dioxide in the liquid phase, to the reactor and/or thecondenser as shown in FIG. 2.

According to a variant shown in FIG. 3, a part of the liquid ammonia isfed with flow line 101 a to power the ejector 118, and a part of theliquid ammonia is fed via flow line 101 b to a mixer 150, wherein saidliquid ammonia and liquid carbon dioxide from line 102 are mixed, andthe resulting liquid mixture is sent to reactor 110 and condenser 114via flow lines 124 and 125. As above, further embodiments of theinvention provides that the mixture produced in the mixer 150 is fed toeither the reactor 110 or the condenser 114 alone; a further input ofgaseous CO2 can also be provided to the reactor 110.

Turning to FIG. 4, an application to a CO2-stripping unit is shown. TheHP loop comprises a reactor 130, a stripper 132 and a shell-and-tubecondenser 134.

A solution comprising urea, carbamate and ammonia is produced in thereactor 130 and is sent to the stripper 132, via a duct 140. Saidstripper 132 is fed with gaseous CO2, which is the stripping agent, byflow line 103 and compressor 137.

Gaseous CO2 acts as a stripping agent, to promote decomposition of theammonium carbamate. Liquid phase from stripper 132, including apartially purified urea solution, is sent to a recovery/finishingsection via flow line 141, while gaseous phase is sent to the condenser134 via line 142.

Gas from top of reactor 130 are sent to a scrubber 136, via line 145;said gas are subjected into said scrubber 136 to absorption with diluiterecycle carbamate solution coming from recovery section via duct 149.

Liquid phase from scrubber 136 is sent to an ejector 135, powered byliquid ammonia entering from line 101; the flow exiting said ejector135, containing the fresh ammonia feed and recycled carbamate, isdirected to condenser 134 via flow line 146.

Liquid and gaseous phase from condenser 134 are separately conveyed toreactor 130 via flow lines 143 (gas) and 144 (liquid).

According to embodiments of the invention, liquid carbon dioxide is fedto either the reactor 130, the condenser 134, or both. FIG. 4 shows anembodiment wherein flow lines 147 and 148 feed part of the liquid CO2 tothe reactor 130, and part of the liquid CO2 to the condenser 134respectively.

Referring to the variant of FIG. 5, a mixer 150 is provided on theliquid CO2 feed line, and liquid ammonia is fed to ejector 135 and tosaid mixer 150 via flow lines 101 a and 101 b. Hence, a part of theliquid ammonia feed is used to power the ejector, and a part is mixedwith the liquid carbon dioxide. The ammonia/carbon dioxide mixtureproduced in said mixer 150 is sent to either the reactor, the condenseror both.

It should be noted that FIGS. 2 to 5 are simplified schemes and detailsand auxiliaries (e.g. pumps, valves, etc.), which are well known to theskilled person, are not shown. It should also be noted that said schemesare open to many variants, also well known in the art.

Referring to FIG. 6, in a preferred embodiment the mixer 150 is a nozzlecomprising an external duct 151 for liquid ammonia and an internal,coaxial duct 152 for liquid CO2. Liquid ammonia flows in the annularspace around duct 152 and is intimately mixed with liquid CO2 exitingthe duct 152. Said duct 152 has an outlet convergent portion 153,corresponding to a convergent portion 154 of the external duct. Saidconvergent portion 154, reducing the cross section of the nozzle,provides acceleration of the liquid flow and improves the mixing ofliquid ammonia and CO2. A mixing portion with decreasing cross sectionis thus obtained between ducts 151 and 152, around and downstream theoutlet 153 of the inner duct.

A following, constant cross-section portion 155 is provided forspreading the flows. Said portion 155 is followed by an outlet divergent156 were mixed flow is slown down.

The nozzle 150 can be installed e.g. upstream the reactor 112 orcondenser 116, so that the liquid ammonia/CO2 mixture is fed to saidreactor or said condenser.

The invention is equally applicable to many different urea plants,including those which can be conduced to the general block diagram ofFIG. 1. The invention is also applicable to modernizing of an existingurea plant, wherein liquid CO2 feeding means are provided in addition toor replacing the existing gaseous CO2 feeding means.

1. A process for producing urea, comprising the steps of: feeding liquid ammonia and carbon dioxide to a synthesis section comprising at least a reactor, a stripper and a condenser forming a high-pressure loop; and reacting said liquid ammonia and carbon dioxide in said synthesis section to produce urea, wherein at least part of said carbon dioxide is fed to said synthesis section in liquid phase.
 2. The process according to claim 1, wherein a part of said carbon dioxide is fed to the synthesis section in liquid phase and the remaining part is fed to the synthesis section in gaseous phase.
 3. The process according to claim 1, wherein the full carbon dioxide input to said synthesis section is in liquid state.
 4. The process according to claim 1, wherein said synthesis section comprises a reactor, a stripper and a condenser, and said liquid carbon dioxide is fed either to said reactor or to said condenser.
 5. The process according to claim 1, wherein said synthesis section comprises a reactor, a stripper and a condenser; a part of said liquid carbon dioxide is fed to the reactor, and a part of the liquid carbon dioxide is fed to the condenser.
 6. The process according to claim 1, wherein said liquid carbon dioxide is mixed with at least part of said liquid ammonia, obtaining a liquid mixture which is fed to said synthesis section.
 7. A plant for producing urea with a process according to claim 1, said plant comprising at least: a synthesis section comprising at least a reactor, a stripper and a condenser forming a high-pressure loop; and feeding means providing an input of fresh ammonia and an input of fresh carbon dioxide to said synthesis section; wherein said feeding means are adapted to feed at least part of said fresh carbon dioxide input to said synthesis section in liquid phase.
 8. The plant according to claim 7, further comprising mixing means disposed to mix said input of liquid carbon dioxide with at least part of said input of liquid ammonia.
 9. The plant according to claim 8, wherein said mixing means comprise a nozzle comprising a first portion with separate coaxial ducts for liquid carbon dioxide and liquid ammonia, and a second mixing portion wherein liquid carbon dioxide and ammonia are mixed.
 10. The plant according to claim 9, wherein said nozzle comprises an external duct and an internal, coaxial duct; the internal duct has a convergent outlet substantially corresponding to a convergent portion of the external duct, obtaining a mixing zone with decreasing cross section in the axial direction, said mixing zone of the nozzle being followed by a constant cross-section portion and a divergent to slow down the liquid mixture.
 11. A method for improving efficiency of a plant for producing urea, said plant being a self-stripping or CO₂-stripping plant comprising at least a synthesis section comprising a reactor, a stripper and a condenser in a high-pressure loop, connected to ammonia feeding means and gaseous carbon dioxide feeding means, said method comprising providing liquid carbon dioxide feeding means connected to said synthesis section.
 12. The method according to claim 11, further comprising mixing means adapted to mix an input of liquid ammonia with an input of liquid carbon dioxide and feed the resulting liquid mixture to said synthesis section.
 13. The method according to claim 11, wherein liquid carbon dioxide feeding means are provided to feed liquid carbon dioxide to said reactor and/or said condenser. 